10.4.2 Choosing a Collation ID

Each collation must have a unique ID. To add a collation, you
must choose an ID value that is not currently used. MySQL
supports two-byte collation IDs. The range of IDs from 1024 to
2047 is reserved for user-defined collations. The collation ID
that you choose will appear in these contexts:

Before MySQL 5.5, which provides for a range of user-defined
collation IDs, you must choose an ID in the range from 1 to
254. In this case, if you upgrade MySQL, you may find that the
collation ID you choose has been assigned to a collation
included in the new MySQL distribution. In this case, you will
need to choose a new value for your own collation.

In addition, before upgrading, you should save the
configuration files that you change. If you upgrade in place,
the process will replace the your modified files.